全文获取类型
收费全文 | 300篇 |
免费 | 26篇 |
国内免费 | 10篇 |
专业分类
化学 | 130篇 |
晶体学 | 4篇 |
力学 | 12篇 |
综合类 | 5篇 |
数学 | 105篇 |
物理学 | 80篇 |
出版年
2023年 | 1篇 |
2022年 | 4篇 |
2021年 | 10篇 |
2020年 | 2篇 |
2019年 | 6篇 |
2018年 | 15篇 |
2017年 | 9篇 |
2016年 | 13篇 |
2015年 | 14篇 |
2014年 | 7篇 |
2013年 | 38篇 |
2012年 | 16篇 |
2011年 | 13篇 |
2010年 | 5篇 |
2009年 | 15篇 |
2008年 | 11篇 |
2007年 | 9篇 |
2006年 | 9篇 |
2005年 | 6篇 |
2004年 | 9篇 |
2003年 | 25篇 |
2002年 | 26篇 |
2001年 | 28篇 |
2000年 | 15篇 |
1999年 | 14篇 |
1998年 | 8篇 |
1997年 | 4篇 |
1995年 | 2篇 |
1992年 | 2篇 |
排序方式: 共有336条查询结果,搜索用时 31 毫秒
1.
Abiola O. Ajayeoba Adewoye A. Olanipekun Wasiu A. Raheem Oluwaseun O. Ojo Ayowumi R. Soji–Adekunle 《声与振动》2021,55(1):69-85
Economic wood processing employs the use of industrial machines for cutting, shaping, milling, and sawing timber, thereby leading to the generation of high levels of noise. Published data from empirical studies have categorized noise as an environmental hazard of global significance. Furthermore, noise exposure limits for different industries and all the industrial machines available has not been formally established as it presently exists in developed nations around the world. Therefore, this study assessed the daily exposure of sawmills workers to noise in Southwestern Nigeria. Reconnaissance surveys were first carried out in Osun, Oyo, Ondo, Ekiti, Lagos, and Ogun States to select sawmills that were fully operational and fit for the study. Two fully functional sawmills in two cities of each State were eventually selected for data collection, making a total of 24 sawmills, while the Circular Machines (CM), Planer Machines (PM), and Band-saw Machines (BM) were the machines in each sawmill considered. Two machines each of CM, PM, and BM were considered in each sawmill, making a total of forty-eight (48) machines each of CM, PM, and BM. Sound data were collected between 7 am and 7 pm each day for six days (between Monday and Saturday) using Extech 407732 sound level meter and all stabilized measurements were taken three times at different intervals. The data collected were in three different periods: Machine No-work Period (NPm), Machine Idle Period (IPm), and Machine Working Period (WPm). A two–way Analysis of Variance (ANOVA) was carried out at P < 0.05 to determine whether there is a significant difference in the sound level average before and after the break, for both the idle and working periods of the three machines considered. This was also done to determine whether there is a significant difference between the sound level average of the results collected during idle and working periods of the three machines. Noise Pollution Levels (Lnp) ranged from 83.20 dB (PM) to 107.65 (BM) and 93.42 (CM and PM) – 116.00 (BM) respectively, while IPm also gave the least noise pollution level of 95.79 dB and WPm gave the highest level of 102.88 dB. The results revealed that all the machines’ Lnp values in the working period are more than the 90 dB acceptable limit the recommendation value of 90 dB while 89.6% of CMs, 75% of PMs, and 89.6% of BM had their Lnp above 90 dB in the idle period respectively. The minimum and the maximum noise dose levels for IPm, WPm and overall are 0.09 (BM) and 2.37 (CM), 0.50 (CM), and 4.77 (PM) and 0.69 (BM) and 6.64 (PM) respectively. The study found out that the fundamental contributing factors to the high noise levels in sawmills are poor machine maintenance, use of old and obsolete machines, poor housekeeping strategy, limited space, workers’ negligence, lack of PPE, and lack of occupational safety training. The study recommends that proper workplace practices such as use of personal protective equipment, new and modern machines, training, and occupational safety programmes be implemented in the considered sawmills. 相似文献
2.
Structure Modification Function of g‐C3N4 for Al2O3 in the In Situ Hydrothermal Process for Enhanced Photocatalytic Activity 下载免费PDF全文
Prof. Fa‐tang Li Shao‐jia Liu Ya‐bin Xue Dr. Xiao‐jing Wang Dr. Ying‐juan Hao Dr. Jun Zhao Rui‐hong Liu Prof. Dishun Zhao 《Chemistry (Weinheim an der Bergstrasse, Germany)》2015,21(28):10149-10159
Heterojunctions of g‐C3N4/Al2O3 (g‐C3N4=graphitic carbon nitride) are constructed by an in situ one‐pot hydrothermal route based on the development of photoactive γ‐Al2O3 semiconductor with a mesoporous structure and a high surface area (188 m2g?1) acting as electron acceptor. A structure modification function of g‐C3N4 for Al2O3 in the hydrothermal process is found, which can be attributed to the coordination between unoccupied orbitals of the Al ions and lone‐pair electrons of the N atoms. The as‐synthesized heterojunctions exhibit much higher photocatalytic activity than pure g‐C3N4. The hydrogen generation rate and the reaction rate constant for the degradation of methyl orange over 50 % g‐C3N4/Al2O3 under visible‐light irradiation (λ>420 nm) are 2.5 and 7.3 times, respectively, higher than those over pristine g‐C3N4. The enhanced activity of the heterojunctions is attributed to their large specific surface areas, their close contact, and the high interfacial areas between the components as well as their excellent adsorption performance, and efficient charge transfer ability. 相似文献
3.
4.
Juan F. Navarro M.C. Martínez–Belda 《Mathematical Methods in the Applied Sciences》2020,43(5):2289-2300
In the N-body ring problem, we investigate the motion of a massless body interacting with N bodies of equal masses at the vertices of a regular polygon that rotates around a central mass. In this paper, we analyze the use of different surfaces of section in the numerical exploration of the escape in the N-body ring problem in order to get some conclusions about the geometry of the basins of escape in the corresponding configuration spaces. 相似文献
5.
Simultaneous determination of aurantio‐obtusin,chrysoobtusin, obtusin and 1‐desmethylobtusin in rat plasma by UHPLC‐MS/MS 下载免费PDF全文
Wei‐dong Zhang Peng‐yuan Wang Ying Wang Qing Wang Yi Gu Jun Cao Shao‐qing Wang Xiao‐juan Wang 《Biomedical chromatography : BMC》2014,28(3):369-374
A sensitive and reliable ultra‐high‐performance liquid chromatography–electrospray ionization–tandem mass spectrometry (UHPLC‐MS/MS) method was developed and validated for the simultaneous determination of four active components of Semen Cassiae extract (aurantio‐obtusin, chrysoobtusin, obtusin and 1‐desmethylobtusin) in rat plasma after oral administration. Chromatographic separation was achieved on an Agilent Poroshell 120 C18 column with gradient elution using a mobile phase that consisted of acetonitrile‐ammonium acetate in water (30 mm ) at a flow rate of 0.4 mL/min. Detection was performed by a triple‐quadrupole tandem mass spectrometer in multiple reaction monitoring mode. The calibration curve was linear over a range of 3.24–1296 ng/mL for aurantio‐obtusin, 0.77–618 ng/mL for chrysoobtusin, 34.55–1818 ng/mL for obtusin and 1.86–1485 ng/mL for 1‐desmethylobtusin. Inter‐ and intra‐day assay variation was <15%. All analytes were shown to be stable during all sample storage and analysis procedures. Copyright © 2013 John Wiley & Sons, Ltd. 相似文献
6.
Mesoporous chitosan‐immobilized iron tetrakis(4‐carboxyphenyl)porphyrin as a model of cytochrome P‐450 enzyme for oxidation of ethylbenzene 下载免费PDF全文
Mesoporous chitosan‐grafted iron tetra (4‐carboxyphenyl) porphyrin catalyst (Fe TCPP/mesp‐CTS) was prepared and investigated as a practical model for the nano‐cavity and coordinate regulation‐catalysis(CRC) function in cytochrome P‐450 enzyme. Fe TCPP/mesp‐CTS was characterized by X‐ray Diffraction (XRD), Thermogravimetry (TG), Ultraviolet–visible spectroscopy(UV‐Vis), Ultraviolet–visible– Diffuse reflectance spectroscopy (UV‐DRS), Scanning electron microscope (SEM), Fourier transform infrared spectroscopy (FT‐IR), and X‐ray photoelectron spectroscopy (XPS) techniques. The catalytic activity of Fe TCPP/mesp‐CTS for ethylbenzene oxidation was investigated and it was proved to be a better catalyst than Fe TCPP/macp‐CTS based on the ethylbenzene conversion, turnover numbers(TON), and the reusability. These results are attributed to the mesocavity and CRC of amino group in Fe TCPP/mesp‐CTS. The highest ethylbenzene conversion and yields of ketone and alcohol were 24.4% and 18.2%, respectively. 相似文献
7.
8.
D. Gopi R. Saraswathy G. Athithya L. Kavitha Jeong‐Heo Bae Dae–Kyeong Kim 《Surface and interface analysis : SIA》2012,44(10):1331-1337
In this paper, we have reported the anti‐corrosion performance of ceria / poly (indole‐co‐pyrrole) (Ce/(poly(In‐co‐Py)) bilayer coating on low nickel stainless steel (LN SS). Electrochemical polymerization of (poly (In‐co‐Py)) was achieved on ceria‐coated LN SS (CeO2/LN SS) in acetonitrile medium containing LiClO4 (ACN‐LiClO4) by cyclic voltammetric technique. The coatings were characterized by analytical techniques like Fourier transform infrared spectroscopy, scanning electron microscopy with energy dispersive analysis of X‐ray, respectively. The mechanical behavior of the coatings was studied using peel test, hardness and wear resistance tests. The corrosion defensive performance of this bilayer coating on LN SS was investigated using electrochemical techniques such as potentiodynamic polarization and electrochemical impedance spectroscopy in 0.5 M H2SO4. These results show that the bilayer coating on LN SS lowered the permeability of corrosive ions present in the acidic medium and thus acts as a barrier against the attack of corrosive environment. Copyright © 2012 John Wiley & Sons, Ltd. 相似文献
9.
Y.–B. Chung 《Mathematical Notes》2017,101(3-4):529-541
We construct an orthonormal basis for the class of square integrable functions on bounded domains in the plane in terms of the classical kernel functions in potential theory. Then we generalize the results of Brown and Halmos about algebraic properties of Toeplitz operators and Laurent operators on the unit disc to general bounded domains. This is a complete classification of Laurent operators and Toeplitz operators for bounded domains. 相似文献
10.
该文基于牛血清白蛋白模板金纳米簇(BSA@AuNCs)与羟基氧化钴(CoOOH)纳米片构建了一种激活型荧光纳米探针用于生物硫醇的检测。带负电的BSA@AuNCs能通过静电吸附作用组装到带正电的CoOOH纳米片表面,与此同时,BSA@AuNCs的荧光由于内滤效应(IFE)有效地被CoOOH纳米片猝灭,形成BSA@AuNCs-CoOOH纳米探针。当向纳米探针溶液加入生物硫醇(0.05~150 μmol/L)时,生物硫醇与纳米探针中的CoOOH纳米片发生氧化还原反应,CoOOH纳米片被降解生成Co2+,同时释放出BSA@AuNCs,BSA@AuNCs荧光信号恢复。结果表明,该纳米探针可以检测低浓度的生物硫醇,对生物硫醇(半胱氨酸、谷胱甘肽和高半胱氨酸)的检出限为30 nmol/L。相对于其他的氨基酸、金属离子及糖类化合物,该纳米探针对生物硫醇具有较高的选择性并成功应用于人血清样品中生物硫醇的检测。 相似文献